In the manufacture of concrete, the concrete is usually cast using a form wherein the concrete takes the shape of the form. The wet concrete is poured into or against the concrete form and, upon setting and removal of the form, the newly-exposed concrete surface is a reverse impression of the inner surface of the form. In the case of wooden forms, the concrete takes the appearance of the wood grain; and in the case of forms involving seamed form members, the concrete shows any seams which have not been sufficiently masked.
Air is often added to a concrete mix and water is often added in excess of the amount required for hydration. Such air and water are useful to render the mix flowable and to facilitate handling and pouring. However, the excess water, if left undrained, results in concrete having a weakened surface and, the air, if not removed, results in surface pores as large as 0.1 to 3 cm, which pores leave an uneven surface open to the effects of dirt and erosion by the freezing/thawing cycles of water.
Efforts have been made in the past to overcome these drainage and removal problems. For example, U.S. Pat. Nos. 5,124,102 and 5,135,692 (both to Serafini) disclose porous fabrics useful for allowing excess water and air to pass therethrough but substantially no cement particles. However, under certain circumstances these fabrics have proven to be sensitive to concrete fluidity and excessive concrete compaction and vibration. If conditions are not well controlled, sufficient cement particles will also pass through the porous fabric, collect on the backside of the fabric sheet, prevent further drainage, and thereby provide diminished concrete properties (e.g., white spots). This condition is particularly prevelant when the concrete form is directly vibrated as is necessary in some cases where normal curing won't suffice. Also, when using these prior art porous fabrics, proper fixation and tensioning of the fabric over the form are essential, not to mention time consuming.
With these prior art fabrics, fine concrete particles typically fill the fabric's larger pores, especially if excessive concrete compaction occurs. Usually, if enough fine concrete particles have entered the fabric structure and sufficient concrete curing is allowed, then the separation of the fabric from the cured concrete becomes very difficult or even impossible. This occurs because the concrete particles that have entered the fabric and hardened therein pull the fabric fibers out of the surface of the fabric when the fabric is separated from the concrete. The problem becomes worse when the fabric is reused with loose surface fibers since the loose fibers tend to become embedded in the cured concrete thereby causing delamination of the fabric sheet. The problem is heightened if the fabric is not handled with care during form assembly and disassembly, since mechanical friction (e.g., rubbing) tends to make the fabric fuzzy and causes the loose fibers to stick to the concrete. Multiple use of the fabric causes more of the fabric pores to become plugged by fine concrete particles resulting in greatly reduced levels of water and air evacuation via draining.
Clearly, what is needed is an improved concrete form and concrete form liner which do not have the deficiencies inherent in the prior art. Specifically, the improved form should allow drainage of excess water from the concrete surface but should prevent concrete particles from passing into the form liner and curing therein. In addition, the form should be useable without form liner tensioning. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached drawings and to the detailed description of the invention which hereinafter follows.